Abstract

Glaciolacustrine rhythmites are being deposited in proglacial Malaspina Lake, Alaska. Fathometer profiles of the lake bottom depict irregular subglacial topography little modified by lacustrine sedimentation and areas of flat fleatureless topography that are underlain by sequences of graded lacustrine sediments. Turbidity currents were recorded along the ice-contact margin of the lake and adjacent to inflowing streams. Cores taken from the lake bottom contain rhythmites whose sandy and silty portions consist of normal and reverse graded laminae. The distribution of sand and silt on the lake bottom indicates that sand and silt content of the rhythmites decreases away from known sediment sources--the streams and the areas where turbidity currents and interflows were observed near the margin of the Malaspina Glacier. The lake bottom topography, the presence of turbidity currents, rhythmites with current-bedded sands and silts, and the sediment distribution on the lake bottom all suggest that the coarse fraction of rhythmites is deposited from turbidity currents. The clay fraction on the other hand is deposited from suspension when the turbidity currents cease. Examination of Fathometer profiles indicates that mass movement of lake bottom sediments is common resulting from the collapse of lake sediments over melting ice. Ice taken from the lake bottom in two cores also suggests that parts of Malaspina Lake are still underlain by glacial ice.